A traveling-wave ring-resonator is often used in a laser which must operate in a single longitudinal mode, with very low noise. A laser incorporating such a resonator is also useful for providing pump-radiation for optical parametric frequency conversion (frequency division).
Typically a traveling wave ring-resonator includes what is usually termed by practitioners of the art as an “optical-diode.” One such an optical-diode includes a crystal of a magneto-optic material such as terbium gallium garnet (TGG) or an element of terbium-doped glass. The magneto-optic material is used as a unidirectional polarization rotator, in conjunction with polarization selective elements in the resonator, to provide a loss in one direction of circulation in the resonator that is greater than the loss in the opposite direction. This differential loss forces lasing to occur in the resonator only in the direction of circulation in which the loss is least
Optical-diodes are most effective in a wavelength range between about 400 nanometers (nm) and 1100 nm. The effectiveness of an optical-diode depends on a so-called “Verdet” constant of the magneto-optic material. This constant defines a degree of polarization-rotation, per unit length of the material, per unit applied magnetic field. The Verdet constant for any given material is wavelength-sensitive, and generally decreases with increasing wavelength. In order to compensate for a lower Verdet constant, the path length in the magneto-optic material must be increased, which increase absorption of radiation in the material. This increases the insertion loss of the optical-diode and reduces the power handling capability of the optical-diode due to heating caused by the absorption. Optical-diodes are commercially available that can operate at wavelengths up to about 2,000 nm, i.e., about 2.0 micrometers (μm). These, however, have an insertion (absorption) loss of about 7% or more, and, because of the absorption, can handle radiation power of only about 20 Watts (W) or less. There is a need for a means for causing unidirectional radiation-circulation in a ring-resonator without the use of a prior-art optical-diode.